Geomorphology 44 (2002) 259–271 www.elsevier.com/locate/geomorph

Modern sedimentation in the Lower Negro River, Amazonas State,

Elena Franzinelli*, Hailton Igreja

Department of Geociences, University of Amazonas, Manaus, 69077-000 Brazil

Received 10 May 2000; received in revised form 26 June 2001; accepted 27 June 2001

Abstract

The Negro River, which flows through the north central Amazon Basin, is one of the largest tributaries of the Amazon. The name ‘‘Negro’’ comes from the colour of its water, which reflects the large quantity of dissolved humic acids and iron oxides that also gives the water its characteristic acid pH. The river is estimated to have the fifth largest water discharge in the world, about 30,000 m3/s. The Negro River is characterized by a high dissolved load but a low energy system. Neotectonics and water quality are the principal factors that control the modern sedimentation in the Lower Negro River. The Lower Negro River is controlled largely by a NW–SE tectonic lineament, that is a segment of a major tectonic transcurrent dextral megasystem of the Amazon Basin. Neotectonism in this area is responsible for the depth of the river and for the occurrence of steep ‘‘fale´sias’’ (cliffs), along some parts of its borders. It also seems that neotectonics have influenced the origin of the Anavilhanas Islands, which are a series of anastomosed, elongated silty clayey channel bars, with internal round or long narrow lakes. The ‘‘igapo´’’, which is the forested area flooded during part of the year, appears to have a neotectonic origin as well. Igapo´s are located on intermediate blocks between the high blocks of the ‘‘terra firme’’ and the low blocks of the channel. The absence of clastic sediments carried in suspension is related to the rare appearance of floodplains, which are limited to very thin layers of fine sediments, located on the abrasion shelfs carved in clastic deposits of the Alter do Cha˜o Formation. Sand bars occur in places along the base of the cliffs and along the edges of the channel system. These sand bars are composed of quartz sand, derived from the reworking of the sand of the Alter do Cha˜o Formation. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Lower Negro River; Amazon Basin; Neotectonism; Black water; Anavilhanas Archipelago

1. Introduction explorers of the Amazon region (von Spix and von Mar- tius, 1823–1831; Bates, 1982; von Humbold, 1829). The Negro River is one of the largest tributaries of Sioli (1954) pioneered scientific studies regarding the Amazon and the largest on the north side. The size, the water quality, soil, vegetation and ecological prob- width and colour of its water caught the attention of the lems of Amazonia with a special reference to the Negro River region. However, modern data on the hydrology, climatology, sedimentology and ecology of the Negro * Corresponding author. River and its basin, are also included in papers that deal E-mail addresses: [email protected], [email protected] with the whole of Amazonia. Sternberg (1950), observ- (E. Franzinelli), [email protected] (H. Igreja). ing the NW–SE rectilinear direction of the Negro and

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0169-555X(01)00178-7 260 E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 other rivers to the east flowing in parallel directions, Colombian Llanos, a flat savanna region bounded to first speculated on the effect of tectonics on the lower the west by the Andes Mountains. Most of the upper course of the Negro River. Franzinelli (1994) empha- course of this river, however, is in the Shield sized the great differences between the Negro and the region, a low lying granitic and granulitic Precambrian Solimo˜es especially regarding water quality, mineralo- craton, covered by rainforest (Fig. 1). The region con- gical characteristics, rates of transported load and com- sists of a wide pediplain, between 80 and 160 m above position and texture of modern deposits. sea level. On this plain there are numerous inselbergs Franzinelli et al. (1999) showed that the modern whose tops occur at two levels, about 460 and 700 m. neostructural directions that control the fluvial mega- The Negro River and its tributaries are well entrenched system of the Amazon Basin can be summarized as into the truncated surface of the Guyana Shield. The follows: (1) Solimo˜es River: N60W; (2) Negro River: majority of rivers and tributaries follow fractures or N45W; (3) : N80E; (4) Madeira River: faults formed by regional tectonism. Most of the tribu- N50E; (5) Taruma˜ River: N10E. These directions are taries have rapids or falls, which are less than 8-m high. the result of transcurrent faults affecting parts of the The middle and lower courses of the Negro River South American Plate under tension caused by the col- developed in the deposits of the Amazon sedimentary lision of the Nazca Plate to the west and the Caribbean basin. This region consists of wide plateaus, upper Platetothenorth,whichbeganintheTertiary(Miocene). Pleistocene in age (Projeto Radambrasil, 1978), that The central belt of the Amazon ecosystem has deve- are composed of interfluvial tabular areas alternating loped on the Amazonian Structural Province (Palaeo- with hills carved in the clastic deposits of the Amazon zoic sedimentary rocks) situated between the Branco Basin. To the north of this area, a narrow strip of white River and Tapajo´s River Structural Provinces, north sandstone and yellow siltstone of the Silurian Trombe- and south respectively, on Precambrian igneous and tas Formation. (Caputo et al., 1972) occurs in contact metamorphic rocks) (de Almeida, 1977). These pro- with the granulitic rocks of the Guyana Shield. To the vinces have undergone cyclic tectonic reactivation. south Cretaceous deposits of the Alter do Cha˜o For- The aim of this paper is to discuss the sedimentation mation and Tertiary deposits of the Solimo˜es Forma- that currently occurs in the Negro River, and to dem- tion outcrop. The Cretaceous continental deposits (red onstrate how the modern deposition and water quality clayey sandstones, siltstones and claystones of the in this river system are controlled by regional neotec- Alter do Cha˜o Formation with some silicified beds in tonism. the upper portion) cover the older sedimentary rocks of the basin. The Cenozoic fine gray sandstones and clay- stones of the Solimo˜es Formation cover the Alter do 2. Regional setting Cha˜o Formation in the western part of the Amazon Basin. The drainage area of the Negro River is about The climate of this region is tropical humid. Rainfall 600,000 km2, which covers parts of Colombia, Ven- in the Negro River basin varies from 3500 mm/year in ezuela and Brazil. The Negro River joins the Solimo˜es the upper basin to 2137 mm/year in the lower basin close to the city of Manaus, forming the Amazon (Fig. (Mota, 1996). The average temperature of the region 1). Based on its average discharge, the Negro ranks as varies from 24 to 32 °C. The vegetation of the region is the fifth largest river in the world (Meade et al., 1991). predominantly tropical forest. Its sources are located in the llanos of Colombia where According to the geology of the area, the course of it is called the Guainia. It receives the designation the Negro River can be divided into three parts: ‘‘Negro’’ after its with the Brazo Cassi- (1) The Upper Negro River, from its sources to the quiare, in . The Cassiquiare connects the up- locality of Santa Isabel, where the river leaves the crys- per course of the Negro River with the Orinoco Basin. talline rocks of the Guyana Shield and flows across the It marks the boundary between Colombia and Vene- sedimentary deposits of the Amazon Basin. In this zuela until it enters Brazil at about 1000 km from its reach the channel is straight with slow-flowing sections confluence with the Solimo˜es River. The Upper Negro that alternate with rapids and waterfalls. The maximum River drainage basin is on the sedimentary rocks of the depth is about 12 m in the dry season. E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 261

Fig. 1. A—The Amazon Basin and Negro River. B—Geological map of the Lower Negro River Region.

(2) The Middle Negro River flows on the Tertiary bars, especially near its southern end at the confluence sedimentary deposits, in a general NW–SE direction. with the Branco River, where the crystalline rocks of This reach has a very large channel, with numerous the basement outcrop in a small area and force the river 262 E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 into a narrow channel. In the reach upstream from the sult of the third set of faults, together with the first and Branco River, where it reaches a width of 20 km and second sets forming a NE–SW distension and a NW– more, and a maximum depth of 18 m in the dry season, SE oblique transtension confining the Negro valley, it resembles an active sedimentation basin, with the influence the inflow/evaporation balance of the river. crystalline rocks at the mouth of the Branco River The formation of these wide lake-like features involved forming a barrier causing an upstream accumulation the inclination and rototranslation of blocks, which of sediments (Franzinelli and Latrubesse, 1998). tended to form wide depressions along the main faults (3) The lower course of the Negro River (Fig. 1B), that are concentrated along the riverbed. These dextral between 62°W and its confluence with the Solimo˜es transtensional processes resulted in negative flower (60°W), displays tectonic control. According to Fran- structures (a flower structure is similar in form to a zinelli and Igreja (1990), this region is a structural flower with various-sized overlapping blocks (petals) lineament trending NW–SE, confining the river. The radiating outward and upward from a central fault lineament belongs to a larger transcurrent system of (stem)), from meters to kilometers in size, the petals geological features (faults, folds and rhombochasms) of which form many collapsed structures, which are that occur in the whole Amazon Basin (Fig. 2). In this prominent at the entrance of the Solimo˜es River Basin section the river is up to 20-km wide and it is bordered (Igreja, 1999). by discontinuous cliffs (fale´sia), formed by the Alter The tectonosedimentary evolution of the Lower do Cha˜o deposits. Negro River with an auto-cycling tendency is similar to the processes of politaxitic and oligotaxitic alterna- tion observed in some Cenozoic basins around the 3. Neotectonism world (Fischer and Arthur, 1977; Miall, 1990) that have an accelerated subsidence and an alternation of a A neotectonic model for this region has been pro- population explosion and high biodiversity during the posed based on remote sensing and verification of neo- rainy season, and decimation and low biodiversity dur- tectonic features (Fig. 2) in the field (Franzinelli and ing the dry season. Igreja, 1990). The Lower Negro River region is governed by two distinct neostructural domains that control its physio- 4. Hydrology graphy: (1) normal fault domain and (2) dextral fault domain. The normal domain, located on the left side of The name ‘‘Negro’’ comes from the colour of its the river, maintains and directs the flow of the major water, caused by the large quantity of dissolved humic tributaries (NE–SW) from the east towards the west, acids and iron oxides, which also give the water its and the smaller NW–SE oriented tributaries, which characteristic acidic pH. According to Bringel et al. flow southwest as well as towards the southeast. The (1999), the pH value of the Negro River varies from 4.2 dextral fault domain, consisting of dextral transcurrent for the water of the central channel to 5.8 for the water faults N45W and secondary faults arranged en echelon close to the confluence with the Branco River. The hu- in the direction N70E, promotes the fluvial flow pre- mic acids come from organic material, such as roots, dominantly from west to east. leaves and wood that decay slowly in the water. The These two principal domains, which have formed a headwaters of the numerous small tributaries generally half graben, have been altered by the younger third set drain wide, shallow swamps situated on the acid rocks of faults in a general west–east orientation, resulting in of the Guyana Shield or on sandy sheets derived from a transtensional basin (rhombochasm). This third set is the weathering of these rocks. According to Sioli difficult to identify on radar and satellite images in the (1991), the origin of black water is largely related to north part of the area, but is fairly easy to observe in the the podzolic soil of the headwater area. In this type of field. It is essentially a set of dextral transcurrent faults soil, the organic material rapidly undergoes decompo- and governs the southern part of the region, where it sition to humus, which is easily dissolved by the perco- restricts the channels of both the Negro and Solimo˜es lating rain water and transported to the water table, rivers. Very wide reaches of the river, which are the re- finally emerging on the surface as small streams. E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 263

Fig. 2. Neotectonic model of Lower Negro River, principal structural lineaments: A= High Block, B = Low Block; Strike-slip movements; 555 Significant reverse component; ??? Significant normal component (from Franzinelli and Igreja, 1990). This region is a smalll sub- system of the transcurrent Amazon mega-system.

The mean annual discharge of the Negro is about of Manaus is about 14 m (Trisciuzzi, 1979). The height 30,000 m3/s (Meade et al., 1991). The maximum of the water during the maximum annual flood reaches annual fluctuation in river level measured at the port about 28 m above sea level at the port of Manaus. Ac- 264 E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 cording to Sternberg (1998), the maximum fluctuation Solimo˜es River. The direction of flow of this tributary of this century occurred in 1953, when the level of the depends on the timing of the Solimo˜es or Negro River water reached 29.50 m above sea level. During the floods. For example, when the Negro River is in flood rainy season the high water level facilitates the devel- stage and its water level is higher than the Solimo˜es, opment of a high amount of biodiversity. During the the direction of flow is from the Negro to the Solimo˜es. dry season the low water level results in a decrease of When the Solimo˜es is in flood stage, the flow is biodiversity due to hydrogeochemical factors. reversed. When the Ariau drains into the Negro River Sediment load transported by the Negro River is the sediments that the river transports are deposited at mostly dissolved load. According to Martinelli et al. its mouth, forming an elongate delta (Fig. 7). X-ray (1988), the Negro River supplies an annual total load of analysis of the delta sediments (Franzinelli et al., 1996) 0.057 108 t. The annual total load at Obidos for the confirmed their origin from the Solimo˜es floodplain. Amazon River was estimated to be 2.46 108 t. Ac- cording to Dunne et al. (1998), 7% of the mean annual 5.2. Sand bars load is made up of sand, and 93% of silt and clay. The Negro River carries very little sedimentary load to the Modern sand bars result from the deposition of Amazon. quartz sand along the banks of the river. These are la- teral bars; other types of bars are very rare. Sand bars are more prominent on the south side of the river due to 5. Geomorphology the dominant winds that blow from the northeast ge- nerating waves on the river, which promote accumu- Cross sections from different sites along the Negro lation of the sand along the edges of the tectonic blocks. River have been plotted in Fig. 3. The sections il- These sand bars are typically situated at the foot of the lustrate the control of bedrock and tectonics on chan- cliffs (fale´sias) (Fig. 7) or are isolated along the shores nel morphology. Such features are floodplain, sand bars, where the cliffs have been completely destroyed by Anavilhanas Islands, and igapo´. erosion forming the natural levees of the river. A long, flat sand bar occurs on the right bank downstream from 5.1. Floodplains the confluence with the Branco River. The mean grain size of its white sand is 0.23 mm, the sand is very well Floodplains are very poorly developed along the sorted, and the quartz grains show good roundness. banks of the Negro River because (1) the river is con- This bar is situated in an area of Paleozoic sandy rocks fined in a channel (Fig. 4) formed by faulting (Fran- and reflects the similar textural characteristics of the zinelli and Igreja, 1990), and (2) the river transports source rocks. The best developed sand bars, also called little suspended sediment load. Floodplains are devel- ‘‘beaches’’, are located at Ponta Negra where these are oped above abrasion terraces that are the result of about 2-km long, situated on the north side of the river, erosional activities on the red claystone or silicified 15 km upstream from downtown Manaus and at Praia sandstone of the Alter do Cha˜o Formation. Thin fine Grande, where the bars are about 12-km long, 200-m sandy beds cover these terraces that are completely wide and 16-m high, on the south side, 70 km upstream flooded during the rainy season (Fig. 5). The small areas from Manaus (Figs. 4 and 7). Grain size, sorting and of floodplain are generally limited by cliffs (Fig. 6) roundness analysis indicate that these bars were that can reach 20 m in height and are the edge of the derived from local reworking of sand beds of the Alter ‘‘terra firme’’ or high terrains that are never inundated do Cha˜o Formation (Brito et al., 1994a,b; Brito, 1996). during floods. The cliffs were originated by faults that From a mineralogical point of view, these sands are emphasize the confinement of the river. These small very pure quartz sand (Potter and Franzinelli, 1985). areas of floodplain are more common on the south side of the Negro due to the regional tilting of the neo- 5.3. Anavilhanas Islands tectonic blocks. One of these blocks contains the Ariau River (Figs. 4 and 7), a tributary on the south side The archipelago of the Anavilhanas Islands (Figs. 4, whose upper drainage area is in the floodplain of the 7 and 8) is a prominent feature of the Lower Negro E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 265

Fig. 3. Schematic cross-sections in the Lower Negro River Region. Data from Projeto Radambrasil (1978) and from measurements during fieldwork. Location of the cross sections I, II, and III shown in Fig. 4. K-Cretaceous, Transtensional Fault (Dextral).

River region. It is a complex of numerous silty clayey the extreme end of the islands. The internal lakes in the channel bars each with a core or upstream head that is cores of the islands have less steep edges than the island generally occupied by a lake, and a very long and nar- banks. A section across the tail of the islands displays a row downstream tail. The Anavilhana Island, for sinusoidal topography with crests and lows of the is- example, is 40-km long and 130-m wide, probably lands parallel to the flow. These islands are completely demarcating a fault line of the same dimensions. In the flooded during the rainy season. The islands are built dry season it is possible to see upstream the lateral with yellow-gray clay and silt, and very rarely contain banks, which are very steep. Downstream the banks of finely laminated, reddish mottled fine sand. The lith- the islands can be steep, but generally they terminate ology of these islands shows that their origin is not gradually. The vegetation zonation is marked by a compatible with the actual dynamic conditions of the decreasing height and a transition from igapo´ (periodi- river, which in its present state hardly transports any cally inundated forest) to mangrove type vegetation, at material in suspension. Three elements were essential 266 E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271

Fig. 4. Location map of the cross sections and of the principal modern sedimentary deposits (6) and neotectonic blocks. for their formation: (1) a sufficient amount of clay and Islands present the same clayey content as of all tribu- silt, (2) a very low energy environment, and (3) the li- taries and not only of the Branco River. These new data near accommodation space. According to Sioli (1991), indicate that the sediment that forms the islands did not these very long bars originated by the deposition of the come from the Branco, but from the tributaries that fine sediments that the Branco River supplies to the flow into the depression (rhombochasm). Negro River. Bringel et al. (1999) showed some signi- ficant textural and mineralogical differences between 5.4. Igapo´s (1) the bottom sediments of several small lakes that re- ceive water from the Branco River and (2) that of the Igapo´s are another distinctive depositional feature lakes along the Negro River far from the influence of of the Negro River (Fig. 9). An igapo´ is a periodically the Branco. For example, the composition of the bot- inundated forest, according to Prance (in Sioli, 1984), tom sediments of the lakes under the influence of the inundated by regular annual flood cycles of black water of the Branco River is more sandy (53%) than of water rivers. Igapo´ regions are believed to derive from those downstream in the Negro River. The Anavilhanas low dry land areas (‘‘terra firme’’), which developed E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 267

Fig. 5. Lower Negro River, south bank upstream from Praia Grande Beach bar. Sandy floodplain during flooding. Note the water mixed with sand on the shore due to wave agitation. In the distance, the thin white line is a sandy lateral bar below the terrace of the Alter do Cha˜o deposits. during the Pleistocene or even the Tertiary (Irion et al. etation that grows in these areas. Based on intensive in Sioli, 1984). Igapo´ deposits normally consist of studies of igapo´ areas by remote sensing and verifi- clay, silt, rare fine sand, and a large quantity of cation of geological and structural features in the organic material derived from the decay of the veg- field, Igreja et al. (1998) concluded that the develop-

Fig. 6. Lower Negro River, south bank. Sandy floodplain upstream from Praia Grande Beach bar, distal view. In the background is a cliff carved in the Alter do Cha˜o deposits. 268 E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271

Fig. 7. Anavilhanas Islands in the central part of the Lower Negro River. One can note the lakes, channels and anastomosing islands. Downstream, where the islands form a long tail, the pattern turns deltaic. In the lower right-hand of the figure, on the right bank of the Negro River, the elongate delta of the Ariau River is visible (j), and upstream, on the same bank, the half-moon shaped ‘‘Praia Grande’’ beach is located (P). The tributaries of the right bank have wide mouths as they enter the Negro River, which is a consequence of the control by dextral transcurrent faults. On the left bank, to the north, the Apuau River also indicates the influence of neotectonism in its channel, with a widening in the orthogonal course. (Modified from Folha SA. 20-Z-B, Projeto Radambrasil, 1978). ment of the igapo´s of the Negro River region was a blocks the vegetation of the ‘‘terra firme’’ adapted itself consequence of neotectonics. Specifically, igapo´s are to the aquatic environment. The different elevations of formed by the deposition of sediments and organic the tectonic blocks along the Lower Negro River material above low blocks, sometimes tilted, separated suggest different stages of neotectonic movement by high blocks of the ‘‘terra firme’’. On these low and, in consequence, different stages of adaptative E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 269

Fig. 8. Anavilhanas Islands in the central part of the Lower Negro River. Note the linear arrangement of the bars and the long internal lakes. The flow is right to left (photo by Antonio Iaccovazo). evolution of the ‘‘terra firme’’ species composition into the deposition of the sediments starts upstream and igapo´ species. forms two bars, which act as levees and in some places deflect the flow of water to form secondary channels. Very fine material in suspension is deposited in the 6. Sedimentation in the Lower Negro middle in the internal lakes. Third, downstream, in the direction towards the restraining zone the bars no The description of the neotectonic and geomor- longer have any internal lakes. They taper towards phological features and sedimentological and hydro- the end, giving to the set an aspect of deltaic config- logical aspects confirm that the Negro is an unusual uration. tropical river. The Negro can be divided into three sec- Presently, the provenance of the sediments of the tions (Fig. 1A). First, downstream of the confluence bars cannot be explained, nor the chronological date with the Branco River, it runs straight in the NW–SE of their formation. The Negro River does not have a direction in a narrow channel 5–8-km wide. Second, high sedimentary load due to the nature of weathering when the river enters the Cretaceous deposits the width of the rocks and the geomorphic aspects of the catch- of the channel first increases to 20 km for about 100 km ment area (Stallard and Edmond, 1983; Stallard, then it narrows downstream in the restraining zone 1985). The amount of sedimentary material that the (Fig. 2). Neotectonics deeply influence this portion of Branco River contributes to the Negro in the rainy sea- the river, forming a half-graben rift, located in the clas- son is not sufficient to explain the formation of the tic friable deposits of the Alter do Cha˜o Formation. In Anavilhanas Islands. the wide section, the main channel bifurcates, the two Considering the enlargement of the river on entering new channels flowing along the banks of the river, bet- the friable deposits of the Alter do Cha˜o Formation and ween 18- and 22-m deep, surrounding the Anavilhanas the confinement of the river, it seems reasonable to Islands. The major islands appear to follow faults in the suppose that these bars were formed by the reworking river bed along the principal zone of deformation. (Fig. of the previous sediments in the rift, along with the 2). In these islands or channel bars, sedimentation oc- material coming from the Branco River and other tri- curs by lateral accretion in linear deposits. It seems that butaries. However, textural evidence indicates that the 270 E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271

Fig. 9. Cuieiras River, close to its confluence with the Negro River. The white trunks are those of igapo´ trees that have died due to the subsidence of the low block.

lateral bars in this reach were formed by local sand forests may be considered as concurrent features inher- deposited in an aquatic environment and the exposed ent in the geodynamics of a transtensional basin, which sand underwent aeolian transport (Brito et al., 1996). In is a part of the Amazonian megasystem. The Amazonian contrast, the igapo´s seem to have a tectonic origin, and megasystem is in turn a part of a complex transcurrent their sedimentological, depositional and vegetational dextral fault system that covers all of the territory of aspects are characteristic and different from those of the the Amazon Basin. This transcurrent dextral fault sys- floodplain. In spite of its large discharge, the Negro is a tem is a result of the collision of the northern part of low energy river. Erosion rarely happens. The occur- the South American Plate with the Nazca Plate to the rence of ‘‘terras caı´das’’, bank failures due to the in- west and the Caribbean Plate to the north. teraction of the river water and groundwater, very Depositional, morphostructural, and hydrochemical frequent in the Solimo˜es (Sternberg, 1960), is uncom- processes make the Negro River unusual, with uncom- mon. Remarkably, in some periods of the year, gene- mon characteristics associated with a confined tectonic rally at the beginning of the rainy season, due to the basin environment. Similar valleys, on a smaller scale, nonsynchronous arival of high water (Meade et al., may be predicted to occur in other areas with the same 1991), the Solimo˜es River water blocks the Negro neotectonics orientation within the Amazon ecosystem. River exit, damming its water. The Negro is a very un- usual river on many accounts. Acknowledgements

7. Conclusion We thank the CNPq (Government National Coun- cil of Brazil) for the financial support for the field trips Studies of the current sedimentation in the Lower (Grant no. 520104/94-3NV), Dr. Avijit Gupta and Dr. Negro River indicate that its hydrology, geomorphol- Andrew G. Warne for reviews leading to improvement ogy and evolution are strongly controlled by neotecto- of this paper, and Daniel Ribeiro Fernandes for the nic elements. Floodplains, sand bars, islands and igapo´ layout end reproduction of the figures. E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259–271 271

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